Meter Mishaps
Use of Ammeters and Voltmeters
Not really ideal
Some V
(energy) gets
dissipated in
ammeter.
Some current
leaks through
voltmeter.
Potential Divider Circuits
• Sometimes elements on circuit need less than the
total voltage potential (pd) .
• A circuit that can be adjusted to divide the p.d. called
potential divide.
• Which type of circuit divides voltage among
resistors?
• Series circuits divide total input voltage between
resistors.
The total 12-V is divided proportionally by resistors.
12 V
R
2R
4V
8V
Potentiometers, Rheostats,
variable resistors
• Placed in circuit to vary voltage available to load.
Variable R can dim bulb by increasing R using more E
9-V
If R = 40W,
it gets 2/3
or Vtot, or 6
V. Bulb
gets 3V.
If r = 0W, it
gets 0 or 0 V.
Bulb gets 9V.
0 - 40 W
20 W
• Can it make bulb go off?
• What resistance would be needed to extinguish bulb?
Sliding Contact Variable Resistors
• Envision a long resisting wire.
• It’s as if wire is many resistors in series.
• The voltmeter can read the pd around some of the
wire by touching specific points.
• What does it read in this position?
6.0 V
V
Simplest potentiometer has slider contact.
S
2V
• At top (point S) V = 2 V
• Halfway V = 1 V
• At bottom V = 0V.
2V
• This is equivalent.
• Why?
Ex: If there are 30cm above & 70cm below the
contact, what is the voltmeter reading?
• Find the V reading
around 70 cm.
30cm
2V
70cm
• Can set up proportion.
• Partial R = 70 cm
• Total R = 100 cm
• V = 2V x (70/100)
• 1.4 V
Ex 1: If there are 40cm above & 60cm below the
contact, what is the voltmeter reading?
40cm
2V
60cm
• The reading around
60cm.
• Partial R =
• 60 cm.
• Total R =
• 100 cm.
• V = 2V x (60/100)
• 1.2 V
I can add another resistor to the
circuit and determine the
reading on the voltmeter.
Ex 2. The maximum resistance of this variable resistor
is 100 W. It is placed in series with the 100 W R. When
the sliding contact is at point A, what will the
voltmeter read?
• It will read 6V (the variable resistor is bypassed): the
maximum reading of voltage in this circuit.
• What is the reading of the voltmeter when the sliding
contact is moved to B?
We have, in effect, the following situation.
Therefore, the voltmeter will read 3V.
It is not possible to make the reading of the
voltmeter vary from zero up to the full voltage of
the supply using this circuit.
Ex 3: Now all 3 connections of the 100 W variable
resistor are used as below, what will the
voltmeter reading be? X is a wire, the contact is
at B.
• Zero. The contact bypasses the bulb like a
short circuit. The wire x is in parallel with the
bulb. Wire and bulb have 0V.
Ex 4. The variable resistor is 100 W.
Approximately what resistance does it supply
on the circuit below? What will the voltmeter
read?
• 50 W, since resistance is directly
proportional to length.
• 4V
Ex 5:What is the voltage around the bulb?
9V
40 W
20 W
9V
Uses of potentiometers
• To control volume.
• To control lights (dimmer switch).
Kerr 145 potentiometer
•
•
•
•
•
Demo, explanation, problem.
Nov 04 pg 15 - 18
Kerr pg 150 #32.
N10 S2 A3 Circuits.
S. S2 A2. Sketch Circuit.
Use your data booklets to
sketch the circuit with the
potentiometer, voltmeter, and
bulb, using the IB symbols on
the boards.
Common Applications of Potential Divides
Electrical Sensors
• Devices whose resistance changes with changing physical
conditions.
Light Sensor
LDR – light dependent resistor
R decreases when light shines
Burglar Alarm electric eye
Symbol
1. The diagram shows a potential divider circuit.
In order to increase the reading on the voltmeter the
A. temperature of R should be increased.
B. temperature of R should be decreased.
C. light intensity on R should be increased.
D. light intensity on R should be decreased.
D
Thermistor – R varies with T. Usually the R decreases
w/increasing temperature (counter to our wires).
Symbol.
Strain Gauge
When stretched
R increases b/c
it gets longer &
skinnier.
Find symbol put
in notes.
When too much bend R
increases w stretch, goes down
with compression
Using Potential Divide with Sensors
LDR - When light stops shining, its resistance increases
and V2 increases. If it V2 gets high enough, the switch
will activate perhaps putting on lights. The p.d. divide is
needed to create a p.d. to activate switch.
12V
switch
Use of Sensors
Ex: The cell has an emf of 12-V and no internal resistance.
The p.d. required to activate the switch is 5-V. Find the
value of R1 that will cause the switch to activate when the
resistance of the LDR is 200 kW.
switch
Rearrange the potential divide equation:
Vout =
(Vin)
R2
R1 + R2.
solve for R1.
R1 =
(R2)
Vin - Vout
V out.
Solve for R1.
280 k W.
Which would be a good sensor to
use with a fire alarm?
What would be a good use for a
strain gauge?
Hwk. Read Hamper.
115 – 123
do pg 123 #21